Some prior art circuits have extended the operating range of a bipolar switching transistor by connecting a second bipolar transistor in series with the collector-emitter current path of the switching transistor. While the second transistor can turn off to provide emitter open operation of the switching transistor, thereby extending its voltage rating, the switching times of bipolar transistors are relatively long. Circuits have also been devised to decrease the switching time of bipolar transistors. This has been accomplished by using field effect transistors (FETs) to provide drive current for the switching transistors. Of course, the voltage rating of the bipolar transistor is not extended with such an arrangement.
Prior art reference patents are as follows:
U.S. Pat. No. 3,716,723 to Heuner et al discloses two current carrying paths, each including a FET in series with a diode. The diodes are pulled to permit a node to be charged when one transistor is turned on and discharged when the other transistor is turned on.
U.S. Pat. No. 4,042,836 to Compton et al discloses circuitry in which a FET acts as an off-on type switch and is controlled by a driver voltage applied to its gate electrode. The driver circuit includes means for rapidly switching the FET while drawing relatively low current in the off and on states.
U.S. Pat. No. 3,970,869 to Coats, Jr. discloses a computer switching circuit including a FET and a diode connected between a pair of transistors. The second transistor is the switching device and is controlled by FET which, in turn, receives a signal from the first transistor.
U.S. Pat. No. 4,296,336 to Skanadore discloses a bipolar switching transistor which is controlled by a second bipolar transistor. The patent teaches that the geometry of the transistor is such that the maximum distance between the base contact region and any point of the emitter is sufficiently small to prevent the degree of constriction of emitter current which will cause the critical electrical field of the collector region to be exceeded.